MAXI-SSC Results In 7 Years
Hiroshi Tsunemi,1 Hiroshi Tomida,2 Satoshi Nakahira,3 and SSC team

1Department of Earth and Space Science, Osaka University,
1-1 Machikaneyama, Toyonaka, Osaka 560-0043 Japan
2Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency,
3-1-1 Yoshinodai, Chuo, Sagamihara, kanagawa, 252-5210 Japan
3
Human Spaceflight Technology Directorate, Japan Aerospace Exploration Agency,
2-1-1 Sengen, Tsukuba, Ibaraki, 305-805 Japan
E-mail(HT): tsunemi@ess.sci.osaka-u.ac.jp
Abstract
The MAXI SSC is a slit-camera using X-ray CCDs. Its field of view is 1◦ × 90◦ and covers the wholesky. The CCD performance gradually degrades due to the charged particles while it has been working
since its launch in 2009. Due to the small eﬀective area of the slit, the SSC detection limit is higher thanthat of the MAXI GSC whereas its eﬀective energy range is between 0.6 keV and 7 keV. We have observed
both point sources and diﬀuse emission. We will briefly review the SSC performance and its results in this7 years.
Key words: instruments: X-ray CCDs — surveys — X-rays:general

1.

SSC performance in orbit

The SSC is an X-ray slit-camera on board the MAXI(Matsuoka et al., 2009) launched in 2009 (Tsunemi etal., 2010, Tomida et al., 2011). It consists of two identical cameras, SSC-H and SSC-Z, looking at a horizontal
direction and a zenith direction, respectively. Each camera contains 16 CCDs just below the mechanical slit that
limits the field of view (FOV) of the SSC to be 1◦ × 90◦ .
CCD functions in parallel sum mode as one dimensional
imager. Each CCD has its own single-stage peltier cooler
with which it is cooled below -60◦ C when the camera
body is around -15◦ C. The peltier cooler achieves morethan 40◦ C temperature diﬀerence by using 1 A electric
current. The SSC camera body is connected to two radiators through a loop heat pipe. The surface of theradiators is painted in white so as to avoid to be tooshinny.
Figure 1 shows the thermal history of the CCD for
7 years. Almost all the CCDs are kept around -60◦ Cwith almost no degradation in cooling system. We hada CPU trouble in 2013 where SSC-Z was out of control
for almost one year. During this period, CCDs in SSC-H
were kept in cold condition while those in SSC-Z wereleft with no peltier cooling.
Each SSC camera receives Cu-K fluorescence lines ineach CCD. Cu-K lines are generated by charged particles onto the slats collimator made of copper. We canFig. 1. Thermal history of the CCDs on the SSC in 7 years. SSC-Z
was out of order in 2013 when the CCD temperature is that ofthe camera body temperature.

monitor the performance of the CCD by measuring thestandard deviation of the line profile. Figure 2 shows theevolution of the SSC performance for 7 years. We seethat the SSC gradually degrades due to the charged particles. The degradation depends on the geometrical condition inside the SSC. The far-end of the CCD received
a least damage while that just below the slit received thebiggest damage. We should note that the SSC-Z in 2013
showed almost no degradation when its CCDs were left
unbiased at around -15◦ C.

the sources in SSC in soft band and those in ROSAT(0.1-2.4 keV). We see that there is a very clear correlation between them. Some sources, Cygnus X-1, Her X-1
etc. showing strong time variability deviate from a simple correlation. We also find that the detection limit ofthe SSC is a few mCrab.

Fig. 2. Evolution of the CCDs on the SSC-H (top figure) and on theSSC-Z (bottom figure). The Cu-K line profile (standard deviation)is shown as a function of time.

The CCDs of the SSC are designed to be sensitive toX-rays while they are insensitive to visible photons by Alcoat on the imaging area. However, we find that they aresensitive to infra-red, particularly at device edges. The
data obtained at daytime are heavily saturated depending on the sun direction. Therefore, we usually collectdata obtained at night time. This reduces the observation eﬃciency of the SSC by a factor of two.
2. Comparison with ROSAT
2.1. Point source observation
The SSC can cover the energy range between 0.6 and7 keV. The low energy limit is determined by the thermal noise while the high energy limit is practically determined by the Cu-K lines. Tomida et al. (2016) generated the point source catalog using the SSC data between
2010 August to 2014 April and listed 170 sources. They
measured the source intensity below and above 1.85 keV(soft band and hard band, respectively). The soft bandis out of the GSC energy range (Mihara et al., 2011).
There are 32 sources only seen in the soft band whileother 30 sources are seen only in the hard band. The
soft band data are the unique measurement of the entire
sky since the ROSAT all sky survey in 1991 (Voges et al.,1999). Figure 3 shows the intensity correlation between

Fig. 3. Intensity correlation between point sources detected by theSSC and those by ROSAT. Some sources show intensity change
in this 20 years.

2.2. Diﬀuse structure in the entire sky
The SSC covers the entire sky in unbiased scan observation. Since its energy range is below 2 keV, this is also aunique sky map since the ROSAT all sky survey in 1991
(Freyberg et al., 1999). Figure 4 shows the comparison of the sky map between the SSC (0.6-1.0 keV) andthe ROSAT (0.44-1.21 keV). In both maps, bright point
sources are eliminated so that the we can see a diﬀuse
emission structure in detail. The angular resolution ofthe SSC is about a few degrees while that of ROSAT isabout an order of magnitude better. We see that they
are quite similar to each other with taking into accountthe diﬀerence in angular resolution.
3. GRB Nova and CSB
3.1. GRBThe SSC scans the sky in the fan beam according to theorbital motion of the International Space Station orbital
motion. Therefore, a typical on-source time is about
50 seconds at every 90 minutes. We have detected several
short-time transients under this circumstance. Amongthem, we detected the initial phase of GRB100418A fromits very beginning (Imatani et al., 2016). Some GRBs
having a pre-cursor were detected by the BAT. The following observation by the Swift/XRT could cover themain part of it (Romano, et al., 2006). While many ofthem show no pre-cursor whose main peak is diﬃcult tobe observed from its beginning.

Fig. 5. Entire light curve of GRB100418B. The first 50 second wasobserved by the SSC and followed by the Swift/XRT.

Fig. 4. Comparison of the SSC diﬀuse map with that of ROSAT.
With taking into account the diﬀerence in spatial resolution, we
see that they are identical to each other.

Figure 5 shows the light curve of GRB100418A. It wasa typical long GRB. When the source entered the FOVof the SSC, the GRB started that was detected by theBAT on Swift. It reached a maximum flux of a few
ten times brighter than that of the Crab nebula. TheSSC monitored its light curve for the initial 50 seconds.
The Swift/XRT pointed to the source 71 second after its
emergence (Marshall, et al., 2011).
The light curve of the GRB can be well expressed bya power law of time. By adding the SSC data in theprompt emission, we found that the light curve could be
expressed not by a power law but by a combination of
a power law and an exponential component of a decay
time about 30 second. In the first 300 second, the exponential components dominates the emission. We foundthat GRB100418A is a typical long GRB having a very
low value of Ep(≤ 8.3 keV) that is a very rare GRB. Itis also consistent with the Yonetoku-relation (Yonetokuet al., 2010) while it is marginal with the Amati-relation(Amati et al., 2008).
3.2. NovaMAXI detected many transients including nova. Since
the GSC has much better detection eﬃciency, the SSC
can confirm it if it is bright enough. A MAXI nova,
MAXI J0158−744, was discovered in the SMC by theGSC in 2011. The SSC followed the observation about

20 minutes after. It was still bright and we obtained
the X-ray spectrum where we detected the emission lines
from Ne and Mg (Morii et al., 2013) showing the thinthermal origin.
We confirmed that the transient is a nova explosion, on
a white dwarf in a binary with a Be star. An early turnon of the super-soft X-ray source (SSS) phase, the short
SSS phase duration of about one month, and a 0.92 keV
Ne emission line found in the third MAXI scan, suggestthat the explosion involves a small amount of ejecta andis produced on an unusually massive O-Ne white dwarf
close to, or possibly over, the Chandrasekhar limit. We
propose that the huge luminosity detected with MAXIis due to the fireball phase, a direct manifestation of theignition of the thermonuclear runaway process in a nova
explosion.
3.3. Cygnus Super Bubble
There are several large scale structures in soft X-ray bandas shown in figure 4. One of them is the Cygnus Super
Bubble (CSB) that is well-known discovered by HEAO-1
in 1980 (Cash, et al., 1980) and ROSAT also confirmed it.
These observations were done by using a gas proportionalcounter which was diﬃcult to reveal its spectral origin.
Figure 6 shows the SSC map around the CSB where there
are 3 Cygnus point sources and the Cygnus Loop. TheCSB is an extended source of about 15◦ × 25◦ .
We detected emission lines of Ne and Mg as shownin figure 7 which confirms a thin thermal origin rather
than a non-thermal origin. The temperature is about
0.23±0.01 keV with a depleted abundance of 0.26±0.1
solar. The abundance depletion is also known in theCygnus Loop (Miyata et al., 1994) that is close to theCSB. It may reflect the interstellar matter in this region. By adding the ROSAT data at low energy, we also
confirm that the interstellar absorption feature showsno variation within the uncertainties suggesting that theFig. 6. CSB image in the Cygnus region. There are three points
sources (Cyg X-1, X-2 and X-3) and the Cygnus Loop.

CSB is a single unity. The Cygnus OB2 association isdiﬃcult to be an energy source, since it is oﬀ-centered ofthe CSB. With taking into account the total thermal energy, we conclude that it can originate from a hypernova
event rather than a rapid sequence of many supernovae(Kimura et al., 2013).

Fig. 8. SSC band map images of the whole sky

Fig. 7. X-ray spectrum of the CSB. It can be well expressed by a thinthermal origin with a depleted abundance.

4. Diﬀuse structure and Fermi bubble
Figure 8 shows the SSC sky maps in three energy bands:
0.6-1 keV, 1-2 keV and 2-4 keV. The 2-4 keV band map
shows many point sources as well as the galactic ridge
emission that is clearly seen in the GSC map. The 0.61 keV map shows diﬀuse emission in large structure.
Figure 9 shows the SSC color map where red, green
and blue correspond to those of three bands in figure 8.
We see that low energy diﬀuse structures are seen in red,
while point sources along the galactic plane are seen inblue. The most prominent large structure is seen in thegalactic center. We see that there is a large structurein the northern hemisphere, emerging from the galactic
plane along the North Polar Spur and reaches near thenorth pole and goes down to the galactic plane. We
also see the structure in the south hemisphere that isquite compact to that seen in the north. It clearly shows
asymmetry in the north and south structure and clearlyoﬀ-set from the galactic center.
A giant gamma-ray structure was discovered by Fermi
in the energy range of 1-10 GeV (Dobler et al., 2010).
They reported that a feature emerges from the galactic
center and extends 50◦ north and south from the planeof the Milky Way, forming a dumbbell shape. This suggests an explosion from the galactic center. There areseveral reports that the X-ray feature is connected withFig. 9. SSC color map of the entire sky where red, green and blue correspond to the energy ranges of 0.6-1 keV, 1-2 keV and 2-4 keV.

the Fermi bubble. Kataoka, et al. (2013) performed observation using Suzaku. Since it has a relatively small
FOV, of 22′ square, they observed several points aroundthe edge of the Fermi bubble, north and south. They
detected an intensity or temperature jump at the edge
location they selected. The spectrum reported is in thermal origin, rather than non-thermal origin.
Comparing the Fermi bubble map with that of theSSC, we see that there is not so clear correlation between
them. In the north hemisphere, the Fermi bubble corresponds to the relatively weak emission in X-ray. The
X-ray feature seems to surround the Fermi bubble, suggesting the formation of the shock front. In the south
hemisphere, however, the Fermi bubble surely includes
the X-ray bright region.
We have accumulated the spectrum from the extended
region covering the Fermi bubble shown in figure 10. We
find that it can not be non-thermal origin. Although
we have to modify the abundance for Ne and Mg, it is athin thermal origin. This result is consistent with that by
the Suzaku observation (Kataoka et al., 2013), although
Suzaku observed a very small region.
Then, we divide the X-ray structure into several segments; inside the Fermi bubble, outside the Fermi bubble, north, south according by latitude and longitude.
We find that the plasma temperature is almost constant,
0.31 keV whole through the region. The absorption feature, NH , is a reliable distance indicator in X-ray. However, we can not determine NH due to the fact thatour data are insensitive below 0.6 keV. According to theROSAT data, there is an intensity variation at low energy due to the local hot bubble or the solar wind charge

Fig. 10. SSC spectrum around the Fermi bubble region

exchange or whatever. It will be quite diﬃcult to estimate the value of NH even if we are sensitive to low
energy. We need higher energy resolution below 0.6 keV.
Figure 11 shows a simple sphere projection using
JUDO2 (http://www.darts.isas.jaxa.jp/astro/judo2/).
The figure center is selected by the center of the X-raystructure so that almost all the extended emission canbe easily seen here. It is about 20◦ away from thegalactic center and not on the galactic plane. There aremany galactic sources along the galactic plane where
they contaminate the extended emission. Apart fromthem, we see that the soft X-ray emission is well withina circle with a radius of 50◦ . The north polar spur isusing good energy resolution will be able to determine
the value of NH to decide whether the X-ray structureis around the galactic center or is very close to us. We
expect that the near future observation with eROSITA(http://www.mpe.mpg.de/eROSITA) will solve thisproblem.

The SSC operation is supported by many people in
the MAXI team as well as students who work outside
the team at present. We greatly appreciate all the eﬀorts
they have done so far.

Fig. 11. Diﬀuse structure around the Fermi bubble region

relatively bright in the north-east edge. Other thanthat, it reminds us a center-filled SNR with a large
scale. Pink color spots in figure 11 indicate the FOVof the Suzaku observations performed so far, most ofthem are for the specific point sources. Very few arefor the study of the extended structure (Kataoka et al.,2013). They can cover much less than 1% of the X-ray
large structure. We have measured the average emission
integral (EI) of the large structure along the line ofsight. We find EI 1 ∼ 4 × 10−2 cm−6 pc with which
we can estimate the total mass (Mt ) and the thermalenergy (Eth ) as below, where D is the distance to thesource and kT is the temperature.
Mt∼ 6 × 104 (D 2.5) M⊙
1 kpc

Eth∼ 4 × 1052

kT
D 2.5() erg
0.3 keV 1 kpc

The X-ray structure showing circular shape is oﬀ-set
from the galactic center while the Fermi bubble is symmetry on the galactic center. In the northern hemisphere, the X-ray bright region surrounds the Fermi bubble while in the southern hemisphere, the X-ray brightregion is surrounded by the Fermi bubble. This suggeststhat they are accidentally seen in the same line of sight.
If the X-ray structure originates from the galactic center,
Mt is an order of 107 M⊙ and Eth is an order of 1055 erg.
This requires us a very strong AGN activity in ourgalaxy. If the source is well within one kpc, these two values become plausible, supplied by hypernova or a seriesof supernova. The precise distance determination is thekey to solve this problem. The observation below 0.6 keV

References
Amati, L., et al., 2008, MNRAS, 391, 577
Cash, W., et al. 1980 ApJ., 238, L71
Dobler, G., 2010, ApJ., 717, 825
Freyberg, M. J., et al., 1999, HXRA conf. 278
Imatani, R., et al. 2016 PASJ., 68, S19
Kataoka, J., et al. 2013 ApJ., 779, 57
Kimura, M., et al. 2013 PASJ., 65-1, 14Marshall, F. E. et al. 2011 ApJ., 727, 132
Matsuoka, M., et al., 2009 PASJ., 61, 999
Mihara, T., et al., 2011 PASJ., 63S, 623
Miyata, E., et al., 1994 PASJ., 46L, 101
Morii, M., et al. 2013 ApJ., 779, 118
Romano, P., et al. 2006 A&A, 456, 917
Tomida, H., et al. 2011 PASJ., 63, 397
Tomida, H., et al. 2016 PASJ., 68, S32
Tsunemi, H., et al. 2010 PASJ., 62, 1371
Voges, W., et al. 1999 A& A, 349, 389
Yonetoku, E., et al., 2010 PASJ, 62, 1495